CISCO DAYS Interview

8/6/2019 CISCO DAYS Interview

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In transparent bridging, what is the forwarding process?

The forwarding process occurs when a switch receives a unicast frame and has an

entry of the destination address in its MAC table. The switch then forwards the frame

to the interface where that destination address res ides.

In transparent bridging, what occurs during the aging process?

Every time a bridge learns a source address, it time-stamps the entry. When the

bridge sees a frame from this source, it updates the time stamp. If the bridge does

not hear from the source for a specific amount of time (called the aging timer), the

bridge deletes the entry from its MAC address table. This process is the aging

process.What is the default aging time in transparent bridges?

The default aging timer is 5 minutes.

What is the Spanning-Tree Protocol (STP)?

STP is a loop-prevention bridge-to-bridge protocol. Its main purpose is to dynamically

maintain a loop-free network. I t does this by sending out Bridge Protocol Data Units

(BPDUs), discovering any loops in the topology, and blocking one or more redundant

links.

How does STP maintain a loop-free network?

STP maintains a loop-free network by

Electing a root bridge

Electing a root port on each nonroot bridge

Electing designated portsPutting in the blocking state any port that is not a root port or designated port

In spanning tree, what is a Bridge ID (BID)?

A BID is an 8-byte field that is composed of the bridge's 6-byte MAC address and a 2-

byte bridge priority.

What is the default bridge priority in a Bridge ID for all Cisco switches?

32,768

In spanning tree, what is path cost?

Path cost is a calculation to determine the link's bandwidth. It is a value assigned to

each port that is based on the port's speed.

What is the spanning tree path cost for each of the following?

10 Mbps

100 Mbps

1 Gbps

The path costs are as follows:

10 Mbps - 100

100 Mbps - 19

1 Gbps - 4

When calculating a loop-free environment, what four-step decision sequence does

spanning tree use to determine what will be the root bridge and which ports will

forward or block?

The four-step decision sequence that spanning tree uses to determine the root bridge

and which port will forward is as follows:

Step 1. The lowest root BID

Step 2. The lowest path cost to the root bridgeStep 3. The lowest sender BID

Step 4. The lowest port ID

How do bridges pass spanning tree information between themselves?

Bridges pass STP information using special frame called Bridge Protocol Data Units

(BPDUs).

How often do bridges send BPDUs out active ports?

The default time that bridges send BPDUs out active ports is 2 seconds.

Note: All ports on a switch listen for BPDUs in case there is a topology change.

In STP, how is a root bridge elected?

In STP, the bridge with the lowest BID is elected the root bridge. All ports on the root

bridge are placed in the forwarding s tate and are called designated ports.Note: The BID is a 6-byte field that is composed of a default priority (32,768) and a

MAC address. Because all Cisco switches use the default priority, the switch with the

lowest MAC address is elected the root bridge. As a rule of thumb, lower will always

win in spanning tree.

After bridges elect the root bridge, what do they do next?

After electing the root bridge, switches elect root ports. A root port is the port on

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nonroot bridges that is closest to the root bridge. Every nonroot bridge must select

one root port.

How do nonroot bridges decide which port they will elect as a root port?

Nonroot bridges use root path cost to determine which port will be the root port. Root

path cost is the cumulative cost of all links to the root bridge. The port with the

lowest root path cost is elected the bridge's root port and is placed in the forwarding

state.

What is the difference between path cost and root path cost?

Path cost is the value assigned to each port. It is added to BPDUs received on that

port to calculate the root path cost. Root path cost is defined as the cumulative costto the root bridge. In a BPDU, this is the value transmitted in the cost field. In a

bridge, this value is calculated by adding the receiving port's path cost to the value

contained in the BPDU.

If a nonroot bridge has two redundant ports with the same root path cost, how does

the bridge choose which port will be the root port?

If a nonroot bridge has redundant ports with the same root path cost, the deciding

factor is the port with the lowest port ID (port number).

After the root bridge and root ports are selected, the last s tep in spanning tree is to

elect designated ports. How do bridges elect designated ports?

In spanning tree, each segment in a bridged network has one designated port. This

port is a single port that both sends and receives traffic to and from that segment and

the root bridge. All other ports are placed in a blocking state. This ensures that onlyone port on any segment can send and receive traffic to and from the root bridge,

ensuring a loop-free topology. The bridge containing the designated port for a

segment is called the designated bridge for that segment. Designated ports are

chosen based on cumulative root path cost to the root bridge.

Note: Every active port on the root bridge becomes a designated port.

If a bridge is faced with a tie in electing designated ports, how does it decide which

port will be the designated port?

In the event of a tie, STP uses the four-step decision process discussed in Question

30. It f irst looks for the BPDU with the lowest BID; this is always the root bridge. If the

switch is not the root bridge, it moves to the next step: the BPDU with the lowest path

cost to the root bridge. If both paths are equal, STP looks for the BPDU with the lowest

sender BID. If these are equal, STP uses the link with the lowest port ID as the final

tiebreaker.

What are the four spanning tree port states?

The four spanning tree port states are

Blocking

Listening

Learning

Forwarding

Remember that root and designated ports forward traffic and that nondesignated

ports block traffic but still listen for BPDUs.

Important note: There is another port state - Disabled - (No frames forwarded, no

BPDUs heard). If it shows up in the answer options - select it along with the others.

What is the STP blocking state?

When a switch starts, all ports are in the blocking state. This i s to prevent any loops

in the network. If there is a better path to the root bridge, the port remains in theblocked state. Ports in the blocked state cannot send or receive traffic, but they can

receive BPDUs.

What is the STP listening state?

Ports transition from a blocked state to a listening state. In this state, no user data is

passed. The port only listens for BPDUs. After listening for 15 seconds (if the bridge

does not find a better path), the port moves to the next state, the learning state.

What is the STP learning state?

In the STP learning state, no user data is being passed. The port quietly builds its

bridging table. The default time in the learning state is 15 seconds.

What is the STP forwarding state?

After the default time in the learning state is up, the port moves to the forwardingstate. In the forwarding state, the port sends and receives data.

What is STP forward delay?

The forward delay is the time it takes for a port to move from the listening state to

the learning state or from the learning state to the forwarding state. The default time

is 30 seconds.





What is the hello time in STP timers?

The hello time is the time interval between the sending of BPDUs. The default time is

2 seconds.

What is the Max Age timer?

The Max Age timer is how long a bridge stores a BPDU before discarding it. The

default time is 20 seconds (ten missed hello intervals).

What is the default time a port takes to transition from the blocking s tate to the

forwarding state?

The default time a port takes to transition from the blocking state to the forwardingstate is 50 seconds: 20 seconds for Max Age, 15 seconds for listening, and 15 seconds

for learning.

What does STP do when it detects a topology change in the network due to a bridge or

link failure?

If spanning tree detects a change in the network due to a bridge or link failure, at

least one bridge interface changes from the blocking state to the forwarding state, or

vice versa.

WAN

The three WAN connection types available are leased lines, circuit-switched, and

packet-switched. Define the differences between each connection type.

Leased lines are dedicated point-to-point lines that provide a s ingle preestablished

WAN communication path from the customer's network to the remote network. Leased

lines are usually employed over synchronous connections. They are generally expensive

and are always up.

Circuit-switched connections are dedicated for only the duration of the call. The

telephone system and ISDN are examples of circuit-switched networks. Packet-

switched connections use virtual circuits (VCs) to provide end-to-end connectivity.

Packet-switched connections are s imilar to leased lines, except that the line is shared

by other customers. A packet knows how to reach its destination by programming of

switches. Frame Relay is an example of a packet-switched connection.

Define customer premises equipment (CPE), and give an example.

CPE is equipment that is located on the customer's (or subscriber's) premises. It is

equipment owned by the customer or equipment leased by the service provider to the

customer. An example is your router.

What is the demarcation point (demarc)?

The demarc is the point where the CPE ends and the local loop begins. It is the last

responsibility of the service provider and is usually an RJ-45 jack located close to the

CPE. Think of the demarc as the boundary between the customer's wiring and the

service provider's wiring.

What is the local loop?

The local loop is the physical cable that extends from the demarc to the central office.

Describe the central office (CO).

The CO is the telco switching facility that connects the customer to the provider's

switching network. The CO is sometimes referred to as a point of presence. It is the

point where the local loop gains access to the service provider's access lines.

What is the toll network?

All the telco switches, COs, and trunk lines inside the WAN provider's network are the

toll network.

What are synchronous links?

Synchronous links have identical frequencies and contain individual characters

encapsulated in control bits, called start/stop bits, that designate the beginning and

end of each character. Synchronous links try to use the same speed as the other end

of a serial link.

What are Asynchronous links?

Asynchronous links send digital signals without timing. Asynchronous links agree on

the same speed, but there is no check or adjustment of the rates if they are slightly

different. Only 1 byte per transfer is sent.

List some typical Layer 2 encapsulation methods for WAN links.

. High-Level Data Link Control (HDLC)

â€¢ Point-to-Point Protocol (PPP)

â€¢ Serial Line Internet Protocol (SLIP)

â€¢ X.25/Link Access Procedure, Balanced (LAPB)





â€¢ Frame Relayâ€¢ Asynchronous Transfer Mode (ATM)

Describe HDLC.

HDLC was derived from Synchronous Data Link Control (SDLC). It is the default

encapsulation type on point-to-point dedicated links and circuit-switched connections

between Cisco routers. It is an ISO-standard bit-oriented data-link protocol that

encapsulates data on synchronous links. HDLC is a connection-oriented protocol that

has very little overhead. HDLC lacks a protocol field and therefore cannot encapsulate

multiple network layer protocols across the same link. Because of this, each vendor

has i ts own method of identifying the network-layer protocol. Cisco offers a propriety

version of HDLC that uses a type field that acts as a protocol field, making it possible

for multiple network-layer protocols to share the same link.

By default, Cisco uses HDLC as its default encapsulation method across synchronous

lines (point-to-point links). I f a serial line uses a different encapsulation protocol, how

do you change it back to HDLC?

To change a serial line back to HDLC, use the following interface command on the

serial interface you want to change: Router(config-if)#encapsulation hdlc

What i s the Point-to-Point Protocol (PPP)?

PPP is an industry-standard protocol that provides router-to-router or router-to-host

connections over synchronous and asynchronous links. It can be used to connect to

other vendors' equipment. It works with several network-layer protocols, such as IP

and IPX. PPP provides authentication through PAP or CHAP.

Describe X.25/LAPB.X.25/LAPB is an ITU-T standard that has a tremendous amount of overhead because

of its strict timeout and windowing techniques. LAPB is the connection-oriented

protocol used with X.25. It uses the ABM (Asynchronous Balance Mode) transfer mode.

X.25/LAPB was used in the 1980s when WAN links were not as error-free as they are

today. X.25 is a predecessor of Frame Relay. X.25 s upports both switched and

permanent v irtual circuits.

What is Frame Relay?

An industry standard, Frame Relay is a switched data link layer protocol that uses

virtual circuits to identify the traffic that belongs to certain routers. It provides

dynamic bandwidth allocation and congestion control.

Quick Notes - INTERNETWORKING

What are the three layers of the Cisco Hierarchical Model?

The three layers of the Cisco Hierarchical Model are: 1

the access layer

The distribution layer

The core layer

In the Cisco Hierarchical Model, what is the function of the access layer?

Sometimes referred to as the desktop layer, the access layer is the point at which

users connect to the network through low-end switches. Some functions of the access

layer include:

Connectivity into the distribution layer

Shared Bandwidth

MAC Address filtering (switching)

Segmentation

What is the function of the distribution layer in the Cisco Hierarchical Model?The distribution layer is also known as the workgroup layer. It is the demarcation

point between the access and core layers of the network. Its primary function is to

provide routing, filtering, and WAN access. The dis tribution layer determines how

packets access the core, so it is the layer at which to implement policy-based

connectivity. Some functions include the following:

Collection point for access layer devices

Broadcast and multicast domain s egmentation

Security and filtering services such as firewalls and access lists

Providing translation between different media types

Inter-VLAN routing

What is the role of the core layer in the Cis co Hierarchical Model?

The core layer is the backbone of the network. Its main function is to switch traffic as

fast as possible. Therefore, it should not perform any filtering to slow down traffic.The ISO's OSI Reference Model contains seven layers. What are they? Include the layer

number and name of each layer in your answer.

The seven layers of the OSI model are as follows:

Layer 7 - Application layer

Layer 6 - Presentation layer

Layer 5 - Session layer





Layer 4 - Transport layer

Layer 3 - Network layer

Layer 2 - Data link layer

Layer 1 - Physical layer

What are some reasons that the industry uses a layered model?

Here are some reasons why the industry uses a layered model:

It encourages industry standardization by defining what functions occur at each level.

It allows vendors to modify or improve components at only one layer versus rewriting

the whole protocol stack.

It helps interoperability by defining s tandards for the operations at each level.

It helps with troubleshooting.

What does the application layer (Layer 7) of the OSI model do, and what are some

examples of this layer?

The application layer is the layer that is closest to the user. This means that this layer

interacts directly with the software application. The application layer's main function

is to identify and establish communication partners, determine resource availability,

and synchronize communication. Some examples include the following:

TCP/IP applications such as Telnet, File Transfer Protocol (FTP), S imple Mail Transfer

Protocol (SMTP), WWW, and HTTP.

OSI applications s uch as Virtual Terminal Protocol, File

Transfer, Access, and Management (FTAM), and Common Management Information

Protocol (CMIP).

In the OSI model, what are the responsibilities of the presentation layer (Layer 6)?

Give some examples of this layer.

Also known as the translator, the presentation layer provides coding and conversion

functions to application layer data. This guarantees that the application layer on

another system can read data transferred from the application layer of a different

system. Some examples of the presentation layer are:

Compression, decompression, and encryption

JPEG, TIFF, GIFF, PICT, QuickTime, MPEG, and ASCII

What are the functions of the session layer (Layer 5)? Give some examples.

The session layer is responsible for creating, managing, and ending communication

sessions between presentation layer entities. These sessions consist of service

requests and responses that develop between applications located on different

network devices. Some examples include SQL, RPC, NFS, X Window System, ZIP,

NetBIOS names, and AppleTalk ASP.

What is the transport layer (Layer 4) responsible for? Give some examples of transport

layer implementations.

The transport layer segments and reass embles data from upper-layer applications into

data streams. It provides reliable data transmission to upper layers. End-to-end

communications, flow control, multiplexing, error detection and correction, and virtual

circuit management are typical transport layer functions. Some examples include TCP,

UDP*, and SPX.

Note: watch out for end-to-end on communications on the exam! Transport layer.

* Error correction does not apply to UDP - connection-less - unreliable.....

What is flow control, and what are the three methods of implementing it?Flow control is the method of controlling the rate at which a computer sends data,

thus preventing network congestion. The three methods of implementing flow control

are

Buffering

Congestion avoidance

Windowing

Almost certain to be on the exam.

Describe the function of the network layer (Layer 3), and give some examples of

network layer implementations.

The network layer provides internetwork routing and logical network addresses. It

defines how to transport traffic between devices that are not locally attached. The

network layer also supports connection-oriented and connectionless service fromhigher-layer protocols. Routers operate at the network layer. IP, IPX, AppleTalk, and

DDP are examples of network layer implementations.

Are network layer addresses physical or logical?

Network layer addresses are logical addresses s pecific to the network layer protocol

being run on the network. Each network layer protocol has a different addressing







that operate at 10 Mbps?

These physical wiring standards operate at 10 Mbps:

10Base2

10Base5

10BaseT

What are collision domains?

In Ethernet segments, devices connect to the same physical medium. Because of this,

all devices receive all signals sent across the wire. If two devices send a packet at the

same time, a collision occurs. In the event of a collision, the two devices run a

backoff algorithm and resend the packet. The devices retransmit up to 15 times. The

first station to detect a collision issues a jam signal. When a jam signal is sent from a

workstation, it affects all of the machines on the segment, not just the two thatcollided; when the jam signal is on the wire, no workstations can transmit data. The

more collisions that occur in a network, the slower it will be, because the devices will

have to resend the packet. A collision domain defines a group of devices connected to

the same physical medium.

What are broadcast domains?

A broadcast domain defines a group of devices that receive each others' broadcast

messages. As with collisions, the more broadcasts that occur on the network, the

slower your network will be. This is because every device that receives a broadcast

must process it to see if the broadcast is intended for it.

What devices are used to break up collision and broadcast domains?

Switches and bridges are used to break up collision domains. They create more

collision domains and fewer collisions. Routers are used to break up broadcastdomains. They create more broadcast domains and smaller broadcast areas.

How do the different layers of the OSI model communicate with each other?

Each layer of the OSI model can communicate only with the layer above it, below it,

and parallel to it (a peer layer). For example, the presentation layer can communicate

with only the application layer, session layer, and presentation layer on the machine it

is communicating with. These layers communicate with each other using protocol data

units (PDUs). These PDUs control information that is added to the user data at each

layer of the model. This information resides in fields called headers (the front of the

data field) and trailers (the end of the data field).

What is data encapsulation?

A PDU can include different information as it goes up or down the OSI model. It is

given a different name according to the information it is carrying (the layer it is at).

When the transport layer receives upper layer data, it adds a TCP header to the data;

this is called a segment. The segment is then passed to the network layer, and an IP

header is added; thus, the data becomes a packet. The packet is passed to the data

link layer, thus becoming a frame. This frame is then converted into bits and is

passed across the network medium. This is data encapsulationApplication layer -- Data

Transport layer -- Segment

Network layer -- Packet

Data link layer -- Frame

There is also the Physical Layer -- Bits

What is the difference between a routing protocol and a routed protocol?

Routing protocols determine how to route traffic to the best location of a routed

protocol. Examples of routing protocols are RIP, EIGRP, OSFP, and BGP. Examples of

routed protocols are IP and IPX.

What 3 devices are used to segment a LAN?Router

Switch

Bridge

Quick Notes - CABLING TECHNOLOGY

What is a straight-through cable, and when would you use it?

A straight-through cable is the same at both ends. A straight-through cable uses pins

1, 2, 3, and 6. The send and receive wires are not crossed. You should use a straight-

through cable when connecting dissimilar devices. Examples include connecting PCs to

switches or hubs or a router to a switch or a hub.

What is a crossover cable, and when would you use it?

A crossover cable is a cable that has the send and receive wires crossed at one of theends. On a Category 5 cable, the 1 and 3 wires and the 2 and 6 wires are switched on

one of the cable's ends. You should use a crossover cable when connecting similar

devices, such as connecting a router to a router, a switch to a switch or hub, a hub to

a hub, or a PC to a PC.

Important tip -- Router (think of it as a PC) to PC via 10BaseT (NIC) uses a "crossover

cable". (contradicts the rule)





How do you set up a console session to a Cisco device?

To set up a console sess ion to a Cisco device, you connect a rollover cable to the

console port on the Cisco device. You then connect the other end to your PC and

configure a terminal emulation application to the following com settings: 9600 bps, 8

data bits, no parity, 1 s top bit, and no flow control.

What is the maximum cable length for each of the following?

The maximum cable lengths are as follows:

10Base2 (thinnet) 185 meters

10Base5 (thicknet) 500 meters

10BaseT 100 meters10BaseFL 2000 meters (400 meters in a shared environment and 2000 meters in a

point-to-point environment)

100BaseT 100 meters

What does Base stand for in 10BaseT and 100BaseT?

Base in 10BaseT and 100BaseT stands for baseband. Baseband is a network technology

in which only one carrier frequency (signal) is used.

What is the difference between baseband and broadband?

Baseband is a network technology in which only one carrier frequency is used (such as

Ethernet). Broadband is a network technology in which several independent channels

are multiplexed into one cable (for example, a T1 line).

Quick Notes - ACCESS LISTS

Besides named access lists, what are the two types of IP access lists?

The two types of IP access lists are s tandard and extended.

What criteria do standard IP access lists use to filter packets?

Standard IP access lists filter packets by the source address. This results in the

packet's being permitted or denied for the entire protocol suite based on the source

network IP address.

What criteria do extended IP access lists use to filter packets?

Extended IP access lists fi lter packets by source address, destination address,

protocols, and port numbers.

In what two ways can IP access lists be applied to an interface?

Access lists can be applied as inbound or outbound access lists. I nbound access lists

process packets as they enter a router's interface and before they are routed.

Outbound access lists process packets as they exit a router's interface and after they

are routed.

How many access lists can be applied to an interface on a Cisco router?

Only one access list per protocol, per direction, per interface can be applied on a Cisco

router. Multiple access lists are permitted per interface, but they must be for a

different protocol.

How are access lists processed?

Access lists are processed in sequential, logical order, evaluating packets from the top

down, one statement at a time. As soon as a match is made, the permit or deny

option is applied, and the packet is not applied to any more access list statements.

Because of this, the order of the statements within any access list is significant.

What is at the end of each access list?

At the end of each access list, an implicit deny statement denies any packet not

filtered in the access list.

What are the number ranges used to define standard and extended IP access lists?

The number ranges used to define standard and extended IP access lists are as

follows:

â€¢ Standard IP access lists 1 to 99 and 1300 to 1999â€¢ Extended IP access lists 100

to 199 and 2000 to 2699

When implementing access lists, what are wildcard masks?

Wildcard masks define the subset of the 32 bits in the IP address that must be

matched. Wildcards are used with access lists to specify a host, network, or part of anetwork. Wildcard masks work exactly the opposite of subnet masks . In subnet

masks , 1 bits are matched to the network portion of the address, and 0s are wildcards

that specify the host range. In wildcard masks , when 0s are present, the octet address

must match.

What is the IOS command syntax used to create a standard IP access list?





Here is the command syntax to create a standard IP access list:

access-list access-list-number {permit deny} source-address [wildcard mask]access-

list-number is a number from 1 to 99.

For example:

RouterA(config)#access-list 10 deny 192.168.0.0 0.0.0.255

How can you display all access lists on a Cisco router?

To display all access lists on a Cisco router, use the show access-list command:

RouterA#show access-list Standard IP access list 10 deny 192.168.0.0, wildcard bits

0.0.0.255Extended IP access list 101 permit tcp any any eq www permit udp any any

eq domain permit udp any eq domain any permit icmp any any deny tcp 192.168.10.0

0.0.0.255 any eq wwwRouterA#

Quick Notes - FRAME RELAY

What protocol does Frame Relay rely on for error checking?

Frame Relay does not rely on any certain protocol for error checking. Instead, it relies

on upper-layer protocols to provide error checking. For example, Frame Relay relies on

TCP to provide error checking in an IP network.

At what layers of the OSI model does Frame Relay operate?

Frame Relay operates at the two lower layers of the OSI model (data link and

physical).

What is the difference between switched virtual circuits (SVCs) and permanent virtual

circuits (PVCs)?SVCs are virtual circuits that are dynamically established when data needs to be

transferred and that are terminated when data transmiss ion is complete. SVCs consist

of four s tates: call setup, data transfer, idle, and call termination. PVCs are

permanently established virtual circuits that operate in one of two states: idle or data

transfer. When the PVC is idle, the connection between the DTE devices is still active.

What is a Data Link Connection Identifier (DLCI)?

A DLCI is a number that identifies the logical circuit between the router and the Frame

Relay switch. It is the Frame Relay Layer 2 address. The Frame Relay switch maps

DLCIs between each pair of routers to create a PVC. For IP devices at the end of each

virtual circuit to communicate, their IP addresses need to be mapped to DLCIs. If you

are running Cisco IOS 11.2 or later, mapping is done automatically using Inverse ARP.

DLCIs have local significance. Think of DLCIs as the MAC address of the Frame Relay

network.

What is the committed information rate (CIR)?

The CIR is the committed information rate, by the service provider, in bits per

second, at which data will be transferred. The service provider sends any data in

excess of this rate if its network has capacity at that time.

How does Frame Relay use Inverse ARP?

Frame Relay uses Inverse ARP as a way to dynamically map a network layer address to

a DLCI. With Inverse ARP, the router can discover the network address of a device

associated with a VC.

What is the Local Management Interface (LMI)?

The LMI is a signaling standard between a CPE device (a router) and the Frame Relay

switch that is responsible for managing and maintaining status between the devices.

It is autosensed with Cisco IOS Release 11.2 and later.

In Frame Relay, what is Forward Explicit Congestion Notification (FECN)?

The FECN is the bit in the Frame Relay header that signals to anyone receiving the

frame (switches and DTEs) that congestion is occurring in the same direction as the

frame. Switches and DTEs can react by slowing the rate at which data is sent in that

direction.

What is Backward Explicit Congestion Notification (BECN)?

The BECN is the bit in the Frame Relay header that signals to switches and DTEs

receiving the frame that congestion is occurring in the direction opposite (backward)

that of the frame. I f switches and DTE devices detect that the BECN bit in the Frame

Relay header is set to 1, they slow the rate at which data is sent in that direction.

In the Frame Relay header, what is the discard eligibility (DE) bit?

If congestion i s detected on the Frame Relay network, the DE bit is turned on in the

Frame Relay header. The DE bit is turned on for frames that are in excess of the CIR.

The DE bit tells a switch which frames to discard if they must be discarded.

What is the default LMI type for Cisco routers that are configured for Frame Relay?





The default LMI for Cisco routers configured for Frame Relay is Cisco. If you are

running Cisco IOS Release 11.2 or later, the Cisco router tries to autosense which LMI

type the Frame Relay switch is using. If i t cannot autosense the LMI type, the router

uses Cisco as its LMI type. The three types of LMIs supported by Cisco routers are:

Cisco

ANSI

Q933a

When a router receives LMI information, it updates its VC status to one of three

states. What are these three states?

The three states that a VC uses to update its status are as follows:

Active state The connection is active, and routers can exchange data.Inactive state The local connection to the Frame Relay switch is working, but the

remote router's connection to the Frame Relay switch is not working.

Deleted state Indicates that no LMIs are being received from the Frame Relay switch

or that there is no service between the router and the Frame Relay switch.

How do you enable Frame Relay on a Cisco router?

To enable Frame Relay on a Cisco router, you must first enable the serial interface for

Frame Relay encapsulation with the encapsulation frame-relay interface command:

RouterB(config)#int s 0

RouterB(config-if)#ip address 192.168.1.1 255.255.255.0

RouterB(config-if)#encapsulation frame-relay

The default encapsulation for a serial interface configured for Frame Relay is cisco. If

you are connecting to a non-Cisco router, how do you change the encapsulation type?

If you are connecting to a non-Cisco router in a Frame Relay network, you need to

specify ietf as the encapsulation type:


RouterB(config-if)#encapsulation frame-relay ietf

If you are using Cisco IOS Release 11.1 or earlier, or if you do not want to autosense

the LMI type, how do you define the LMI type on a C isco router?

To define the LMI type on a Cisco router, use the frame-relay lmi-type {ansi cisco

q933a} interface command:


RouterB(config-if)#encapsulation frame-relay

RouterB(config-if)#frame-relay lmi-type ans i

If Inverse ARP is disabled on your router, how do you reenable it?

Inverse ARP is enabled by default on a Cisco router. If it is disabled, reenable it by

using the following command:

RouterB(config-if)#frame-relay inverse-arp [protocol] [dlci]

Supported protocols indicated by the protocol option include ip, ipx, decnet, appletalk,

vines, and xns.

If a remote router does not support Inverse ARP, you must define the address-to-DLCI

table

statically. How do you create these static maps?To define static maps on a Cisco router, use the following command:

RouterA(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf

cisco] [payload-compress packet-by-packet]

where:

â€¢ protocol defines the supported protocol bridging or LLC.

â€¢ protocol-address is the remote router's network layer address.

â€¢ dlci defines the remote router's local DLCI.

â€¢ broadcast specifies whether you want to forward broadcasts over the VC,

permitting dynamic routing protocols over the VC.â€¢ ietf cisco is the encapsulation

type.

What Cisco IOS command displays the LMI traffic statistics and LMI type?

The show frame-relay lmi command displays the LMI traffic statistics and LMI type:

RouterA#show frame-relay lmiLMI Statistics for interface Serial0 (Frame Relay DTE)

LMI TYPE = CISCO Invalid Unnumbered info 0

Invalid Prot Disc 0 Invalid dummy Call Ref 0

Invalid Msg Type 0 Invalid Status Message 0

Invalid Lock Shift 0 Invalid Information ID 0

Invalid Report IE Len 0 Invalid Report Request 0





Invalid Keep IE Len 0 Num Status Enq. Rcvd 1748

Num Status msgs Sent 1748 Num Update Status Sent 0

Num St Enq. Timeouts 0

routera#

How do you display the current Frame Relay map entries and information about these

connections on a Cisco router?

To view the current map entries and information about the connections, use the show

frame-relay map command:

RouterA#show frame-relay map

Quick Notes - ROUTINGHow do OSPF-enabled routers build adjacencies and exchange their routing tables?

OSPF-enabled routers build adjacencies by sending Hello packets out through all OSPF-

enabled interfaces.

If these routers share a command link and agree on parameters s et within their Hello

packets then they become neighbors. If these parameters differ then the routers do

not become neighbors and communication stops.

OSPF routers form adjacencies with certain routers. These routers are determined by

the layer 2 (data link) media type and as soon as the adjacencies are formed each

router sends LSAs (Link State Advertisements) to all adjacent routers. The LSAs

describe the state of each router's links. There are multiple LSA types and a route that

receives an LSA from a neighbor records the LSA in a link-state database and floods a

copy of the LSA to all its other neighbors.

When all databases are complete - then each router uses the SPF (Shortest-Path First)algorithm to calculate a loop-free topology and builds its routing table based on this

topology.

It is important to note that the Hello protocol is bidrectional and is the means by

which neighbors are discovered and acts as keepalives between neighboring routers. It

also establishes and maintain neighbor relationships and elects the DR (Designated

Router) and BDR (Backup Designated Router) to represent the segment on Broadcast

and NBMA (nobroadcast multiaccess) networks.

Note: Hello protocols are sent periodically sent out each OSPF-enabled interface using

IP multicast address 224.0.0.5. The default interval on NBMA (nonbroadcast

multiaccess) networks i s 30 seconds. The default interval on Broadcast, Point-to-

point, and point-to-multipoint networks is 10 seconds.

What are LSAs (link-state advertisements)?

LSAs are sent out all OSPF-enabled router interfaces describing the state of the

router's links. They are also packets that OSPF uses to advertise changes in the

condition of a link or other OSPF routers.

Name two LSA (link-state advertisement) types?

Type 1 LSAs are router LSAs and are generated by each router for the area to which

the router belongs. These LSAs describe the states of the router'links to the area

(area 0 for example) and are flooded within a single area (area 0 for example).

Type 2 LSAs are network LSAs and are generated by the DR (Designated Router) and

the BDR (Backup Designated Router). They describe the routers attached to a

particular network and are flooded within a single area (area 0 for example).

What is the routing metric OSPF is based on?

Bandwith.

Formula: Cost = 100,000,000 / bandwidth in bits per secondsThe cost of a 100 MBbps connection would be:

1000,000,000 / 100,000,000 =1

Based on the schema above -- if adjacencies are established with only with the DR

(Designated Router) and BDR (Backup Designated Router)- what is the circuit count?

Formula:

2(n - 1) where n is the number of routers in the network.

2(5 - 1) = 8 circuits.

A circuit can also be thought of as an adjacency or connection.

Count four going into the DR and 4 going into the BDR for a total of 8.

Note: OSPF avoids synchronizing between every pair of routers in the network by using

a DR and BDR. This way adjacencies are formed only to the DR and BDR, and the

number of LSAs sent over the network is reduced. Now only the DR and BDR have four

adjacencies, and all the other routers have two.

On an OSPF-enabled router -- what is the router ID and where does an OSPF-enabledrouter receive its router ID?

To initialize - OSPF must be able to define a router ID. The most common and stable

source for a router ID is the IP address set on the logical loopback interface that is

always available. If no logical interface is defined -- then the router receives its ID

from the highest IP address on the physical interfaces.

Note: If two loopback addresses are defined -- it will use the highest loopback





address. Think highest logical or highest physical interface address.

Name five OSPF network types:

Broadcast networks: Ethernet/Token Ring. OSPF-enabled routers on broadcast

networks elect a DR (Designated Router) and BDR (Backup Designated Router). All the

routers in the network form adjacencies with the DR and BDR. Note: OSPF packets are

multicast to the DR and BDR.

NBMA (nonbroadcast multiaccess) networks: Frame Relay/X.25/ATM. NBMA networks

can connect more than two routers but have no broadcast functionality. These

networks elect an DR and BDR. Note: OSPF packets are unicast.

Point-to-point networks: A physical DS1 (T1) for example.

Point-to-point networks connect a pair of routers and always becomes adjacent.

Point-to-multipoint networks: Point-to-multipoint networks are a special configurationof NBMA networks in which networks are treated as a collection of point-to-point links.

Point-to-multipoint networks do not elect a DR or BDR. Note: OSPF packets are

multicast.

Virtual links: Virtual links area special configuration that the router interprets as

unnumbered point-to-point networks. The network administrator creates/defines

virtual links.

What is routing?

Routing is the process in which items are forwarded from one location to another.

Routing is a hop-by-hop paradigm.

A Cisco router performs routing and switching functions. Describe what each function

does.Routing is a way to learn and maintain awareness of the network topology. Each router

maintains a routing table in which it looks up the destination Layer 3 address to get

the packet one step closer to i ts destination.

The switching function is the actual movement of temporary traffic through the

router, from an inbound interface to an outbound interface.

What are the three types of routes you can use in a Cisco router?

The three types of routes are static routes, dynamic routes, and default routes.

What i s the difference between static and dynamic routes?

Static routes are routes that an administrator manually enters into a router. Dynamic

routes are routes that a router learns automatically through a routing protocol.

What is a default route?

Also known as the gateway of last resort, a default route is a special type of static

route with an all-zeros network and network mask. The default route is used to route

any packets to a network that a router does not directly know about to a next-hop

router. By default, if a router receives a packet to a destination network that is not in

its routing table, it drops the packet. When a default route is specified, the router

does not drop the packet. Instead, it forwards the packet to the IP address specified

in the default route.

What is a routing protocol?

A routing protocol defines the set of rules used by a router when it communicates with

neighboring routers. Routing protocols listens for packets from other participants in

order to learn and maintain a routing table.

What is administrative distance?

Administrative distance (AD) is an integer from 0 to 255 that rates thetrustworthiness of routing information received on a router from a neighboring router.

The AD is used as the tiebreaker when a router has multiple paths from different

routing protocols to the same destination. The path with the lower AD is the one given

priority.

What are the three classes of routing protocols?

1) Dis tance vector 2) Link-state 3) Balanced hybrid

What is the AD for each of the following?

Directly connected interface 0

Static route 1

EIGRP 90IGRP 100

OSPF 110

RIP 120

External EIGRP 170

Unknown 255

How do distance vector routing protocols function?





Also known as Bellman-Ford-Fulkerson algorithms, dis tance vector routing protocols

pass complete routing tables to neighboring routers. Neighboring routers then

combine the received routing table with their own routing table. Each router receives

a routing table from its directly connected neighbor. Distance vector routing tables

include information about the total cost and the logical address of the first router on

the path to each network they know about.

How do distance vector routing protocols keep track of any changes to the

internetwork?

Distance vector routing protocols keep track of an internetwork by periodically

broadcasting updates out all active interfaces. This broadcast contains the entire

routing table. This method is often called routing by rumor.Slow convergence of dis tance vector routing protocols can cause inconsis tent routing

tables and routing loops.

What are some mechanisms that distance vector protocols implement to prevent

routing loops and inconsistent routing tables?

Here are some of the ways distance vector routing protocols prevent routing loops and

inconsistent routing tables:

â€¢ Maximum hop countâ€¢ Split horizon

â€¢ Route poisoningâ€¢ Holddowns

What is split horizon?

The rule of split horizon is that it is never useful to send information about a route

back in the direction from which the original update came.

What is convergence?

Convergence is when all routers have consistent knowledge and correct routing tables.

What is route poisoning?

With route poisoning, when a distance vector routing protocol notices that a route is

no longer valid, the route is advertised with an infinite metric, signifying that the

route is bad. In RIP, a metric of 16 is used to signify infinity. Route poisoning is used

with holddowns.

What are hold-down timers?

Hold-down timers prevent regular update messages from reinstating a route that

might have gone bad. Hold-down timers also tell routers to hold for a period of time

any changes that might affect routes.

What are triggered updates?

When a router notices that a directly connected subnet has changed state, it

immediately sends another routing update out its other interfaces rather than waiting

for the routing update timer to expire. Triggered updates are also known as Flash

updates.

What is IP RIP?

IP RIP is a true distance vector routing protocol that sends its complete routing table

out all active interfaces every 30 seconds. IP RIP uses a hop count as its metric to

determine the best path to a remote network. The maximum allowable hop count is

15, meaning that 16 is unreachable. There are two versions of RIP. Version 1 is

classful, and version 2 is classless. IP RIP can load-balance over as many as six equal-

cost paths.

What four timers does IP RIP use to regulate its performance?Here are the four timers that IP RIP uses to regulate its performance: â€¢ Route

update timer Time between router updates. The default is 30 seconds.â€¢ Route

invalid timer Time that must expire before a route becomes invalid. The default is

180 seconds.â€¢ Route hold-down timer If IP RIP receives an update with a hop count

higher than the metric recorded in the routing table, the router goes into holddown for

180 seconds.â€¢ Route flush timer Time from when a route becomes invalid to when it

is removed from the routing table. The default is 240 seconds.

How do you stop RIP updates from propagating out an interface on a router?

Sometimes you do not want RIP updates to propagate across the WAN, wasting

valuable bandwidth or giving out valuable information about your internetwork. The

easiest way to stop RIP updates from propagating out an interface is to use the

passive-interface global configuration command.

How do you display the contents of a Cisco IP routing table?

The show ip route command displays the Cisco routing table's contents.

What is Interior Gateway Routing Protocol (IGRP)?

IGRP is a Cisco proprietary distance vector routing protocol. IGRP has a default hop

count of 100 hops, with a maximum hop count of 255. IGRP uses bandwidth and line





delay as its default metric, but it can also use reliability, load, and MTU.

How do you enable IGRP on a Cisco router?

The way you enable IGRP on a Cisco router is similar to the way you enable RIP, except

you specify IGRP as the protocol and add an autonomous system number. For example:

RouterA(config)#router igrp 10 (10 is the AS number)

RouterA(config-router)#network 192.168.0.0



What four timers does IGRP use to regulate its performance?

The four timers IGRP uses to regulate its performance are as follows:

â€¢ Route update timer Time between router updates The default is 90 seconds.

â€¢ Route invalid timer Time that must expire before a route becomes invalid . The

default is 270 seconds.

â€¢ Route hold-down timer If a destination becomes unreachable, or if the next-hop

router increases the metric recording in the routing table, the router goes into

holddown for 280 seconds.

â€¢ Route flush timer[md]Time from when a route becomes invalid to when it is

removed from the routing table. The default is 630 seconds.

Quick Notes - SWITCHINGWhat are three types of LAN traffic?

Unicasts - intended for one host.

Broadcasts - intended for everyone.

Multicasts - intended for a only a subset or group within an entire network.

What are unicast frames?

Unicast frames are the most common type of network traffic. A unicast frame is a

frame intended for only one host. The only station that processes this frame i s one

station that has its own MAC address in the destination portion of packet.

What are broadcast frames?

Broadcast frames are frames intended for everyone. Stations view broadcast frames

as public service announcements. All stations receive and process broadcast frames.

In large networks, broadcasts can cause serious performance degradation in network

hosts - (broadcast storm).

The destination address of broadcast frames (Data Link / Layer 2 broadcast addresses

is FF-FF-FF-FF-FF-FF or alternatively all 1s in binary (11111111).

What are multicast frames?

Multicast frames address a group of devices that have a common interest. These

frames allow the source to send only one copy of the frame on the network even

though it is intended for several stations. Only stations that have a card that is

configured by software to receive multicast frames for a particular multicast group

can process a frame to that multicast address - all other stations discard multicast

frames

What three major functions do Data Link Layer / Layer 2 Switches perform?

Address learning

Packet forwarding/filtering

Loop avoidance by spanning tree

What will occur when you attempt to segment a network with hubs and repeaters?

Basically, hubs and repeaters become extensions of the physical cable plant. All

devices that connect to either a hub or a repeater share the same bandwidth and by

definition hubs and repeaters create a s ingle broadcast and collision domain.

Think of both devices are pass-through devices much like a electrical power-strip.

Hubs and repeaters res ide on the Physical Layer / Layer 1 of the OSI model where they

pass 0s and 1s along the wire or up to the Data Link Layer. CSUs / Channel Service

Units fall into the same category. All are regarded as unintelligent devices. No

addressing takes place on the Physical layer.

What three devices are used to segment a LAN?

Router - logical addressing - IP address

Switch - physical addressing - MAC address

Bridge - physical addressing - MAC addres

What is microsegmentation?

Each workstation or network device on the network has its own dedicated segment -

also known as a Collision Domain - to a switch. Each device gets the segments full

bandwidth and does not have to share the dedicated segment with other devices.

Collisions are reduced because each segment is its own Collision Domain.





Posted by "CISCO BOY" at 7:22 AM

Labels: CCNP

Important: Full-duplex transmission is achieved by microsegmentation. Each device

can send and receive at the same time which doubles the amount of bandwidth

between nodes.

What are the three switching methods in Cisco Catalyst switches?

The three frame operating modes to handle frame switching are

Store-and-forward

Cut-through

Fragment-free

What is the Cisco Catalyst store-and-forward switching method?

In the store-and-forward switching method, the switch receives the entire frame

before it forwards it. The switch reads the cyclic redundancy check (CRC) to makesure the frame is not bad. If the frame is good, the switch forwards it. Because the

switch stores the frame before forwarding i t, latency is introduced in the switch.

Latency through the switch varies with the size of the frame.

What is the Cisco Catalyst cut-through switching method?

In cut-through switching mode, the switch only checks the frame's destination address

and immediately begins forwarding the frame out the appropriate port. Because the

switch checks the destination address in only the header and not the entire frame, the

switch forwards a collision frame or a frame that has a bad CRC.

What is the Cisco Catalyst fragment-free switching method?

Also known as modified cut-through, fragment-free switching checks the first 64 bytes

before forwarding the frame. Ethernet specifications state that collisions should be

detected during the first 64 bytes of the frame. By reading the first 64 bytes of theframe, the switch can filter most collisions, although late collisions are still poss ible.

What is the default switching mode in Cis co Catalyst 1900 switches?

The default switching mode for the Catalyst 1900 is fragment-free.

What is half-duplex transmission mode?

Half-duplex transmiss ion is the default mode that Ethernet functions in. In half-duplex

transmission, a device can only send or receive--not do both at once. In half-duplex

mode, stations are susceptible to collisions, and efficiency is rated at 50 to 60

percent.

What is full-duplex transmission mode?

In full-duplex mode, a station can send and receive at the same time. In full-duplex

mode, collision detection is disabled. This mode offers 100 percent efficiency in both

directions.

On a Cisco Catalyst 1900 switch, what are the default duplex settings for

10BaseT/100BaseT ports, default switching mode, and default protocols?

What are dynamic addresses on a Catalyst switch?

Dynamic addresses are addresses that the switch learns about dynamically through the

learning process. If the switch does not see a MAC address for a certain amount of

time, it drops the MAC address.

What are permanent MAC addresses on a Catalyst switch?

Permanent MAC addresses are entered manually by the administrator and are not aged

out.

25 comments:

Hugo said...

I recently came across your blog and have been reading about data

link control. I thought I would leave my first comment. I dont know

what to say except that I have enjoyed reading. Nice blog. I will keep

visiting this blog very often.

July 31, 2009 7:09 AM

"CISCO BOY" said...

August 2, 2009 10:32 AM

Anonymous said...

hi, this blog has become on of my fav ones....i visit this blog very





often....thanx for such a good blog....

can I know any exceptional cases in BGP and any good interview

questions on BGP and MPLS VPN

THANK YOU.

August 20, 2009 3:31 PM

Anonymous said...

[email protected] blog is very useful.

Thank you.

January 20, 2010 10:13 PM

Rankeshwar said...

Simply Awesome

Loving it

February 3, 2010 10:42 AM

Anonymous said...This is a great blog. Thank you for taking the time and brief all these

topics with us.

Regards, AM

March 21, 2010 7:28 PM

Anonymous said...

Great Efforts by the Author.

Very Thanking you

June 7, 2010 9:50 PM

Anonymous said...

Great blog to share the kb..Excellent effort. .!!

June 9, 2010 4:39 AM

Lucien said...

Excellent effort bro. Very much appreciated..

June 18, 2010 2:10 PM

Sandip said...

Really fantastic effort, excellent !

July 3, 2010 9:32 AM

Sandip said...

Really fantastic effort, thanks for the excellence !

July 3, 2010 9:35 AM

purshoth said...

superB..

July 6, 2010 3:12 AM

Anonymous said...

nice very helpfull

July 13, 2010 9:29 PM





Anonymous said...

Gr8 work...many thanks

August 28, 2010 10:23 PM

Anonymous said...

thnk you so much .... for such a hard work.... 5/5

ur work made my day.... <3 <3 <3

September 11, 2010 8:53 PM

Brice said...

perfect!!

September 27, 2010 5:14 AM

robinho said...

Hi

Tks very much for post:

I like it and hope that you continue posting.

Let me show other source that may be good for community.

Source: Difficult interview questions

Best rgs

David

October 11, 2010 5:25 PM

Mohamed shareej said...

nice

October 23, 2010 7:38 AM

Anonymous said...

This became my lifeline to my 2mrw's interview...

Thanks allot...

-Rakesh

October 23, 2010 12:27 PM

muhammad said...

hi .Thats nice but un compleat.plz add also .information question onbgb mpls and ospf.

all oves is thats very nice

November 4, 2010 10:05 AM

TIRUMALA said...

Hi Thnks alot for the posting the different types of questions.

Thanks alot

Regards

Varun


Darko said...

"The BID is a 6-byte field that is composed of a default priority

(32,768) and a MAC address."





BID is a 8 byte field composed of 2 byte priority and 6 byte MAC

address.


Darko said...

"What is STP forward delay?

The forward delay is the time it takes for a port to move from the

listening state to the learning state or from the learning state to the

forwarding state. The default time is 30 seconds."

Default time for STP forward delay is 15 seconds.


davi said...

Hi

I read this post 2 times. It is very useful.

Pls try to keep posting.

Let me show other source that may be good for community.

Source: Transportation interview questions

Best regards

Jonathan.

December 27, 2010 1:51 AM

Anonymous said...

Some teams are horrible in Cisco, worst than Samsung. especially

teams in VTV, CISCO, India.

John Chambers is sleeping and some junk managers are running the

teams.There is group of karthik Ramachandran in VTV Bangalore india

office. This team is horrible. Managers suck the blood of employees.

24 hours work, managers shout over people, crazy managers and

work estimate is always asap. Never even give the interview in this

team. Here Managers do not let people go home and people even

sleep and live in office and are not allowed to go home. it is a slavery

in this group.

So friends slavery is still alive in Cisco India office, not all but

definitely some and Mr Chambers doesnot know what kind of

company he is running where in India people are tortured worst than

when India was not independent.

Such Managers should be sent to kept as slaves!

February 15, 2011 10:00 AM

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